Ammonium-nitrogen contained in wastewater is one of causative agents of eutrophication in such as river, lake, and sea and has to be sufficiently removed. Generally, ammonium-nitrogen in wastewater is converted into nitrogen gas through a two-staged biological reaction process consisting of a nitrification process and a denitrification process. In the nitrification process, ammonium-nitrogen in the waste water is oxidized into nitrite-nitrogen by ammonia-oxidizing bacteria and a part of thus obtained nitrite-nitrogen is further oxidized into nitrate-nitrogen by nitrite oxidizing bacteria. In the denitrification process, the nitrite-nitrogen and the nitrate-nitrogen are converted into nitrogen gas by denitrifying bacteria as heterotrophic bacteria, using organic materials as an electron donor.
However, the conventional nitrification-denitrifying method has a shortage of high running cost because it requires organic materials such as methanol as an electron donor in the denitrification process and large amount of oxygen in the nitrification process.
Recently, a method for denitrification by reacting ammonium-nitrogen and nitrite-nitrogen using autotrophic microorganisms (hereinafter, sometimes referred to as “ANAMMOX bacteria”) with the ammonium-nitrogen acting as an electron donor and the nitrite-nitrogen acting as an electron acceptor has been proposed. This method does not require addition of organic materials and is able to lower the cost compared to the conventional method using heterotrophic denitrifying bacteria. In addition, the autotrophic microorganisms generate lower amount of sludge than that of the heterotrophic microorganisms because of lower yield of the autotrophic microorganisms, thereby reducing the amount of excess sludge compared to the method utilizing heterotrophic denitrifying bacteria. Moreover, N2O observed in case of the conventional nitrification-denitrifying method is not generated, thereby reducing load on the environment.
The biological denitrification process utilizing ANAMMOX bacteria was reported in Strous, M, et al., Appl. Microbiol. Biotechnol., 50, p. 589-596 (1998). It is believed that ammonium-nitrogen and nitrite-nitrogen are decomposed into nitrogen gas in the following reaction:1.0NH4++1.32NO2−+0.066HCO3−+0.13H+→1.02N2+0.26NO3−+0.066CH2O0.5N0.15+2.03H2O  (1)
To conduct the biological denitrification treatment using the ANAMMOX bacteria, it is required to conduct nitrite type nitrification in which ammonium-nitrogen in wastewater is treated with ammonia-oxidizing bacteria in such a manner that the oxidation is stopped when the ammonium-nitrogen is oxidized into nitrous acid before the ammonium-nitrogen is oxidized into nitric acid.
It is known that nitrite type nitrification reaction could be achieved by controlling the DO (dissolved oxygen) concentration to be kept low. In other words, by supplying merely required amount of oxygen to oxidize ammonium-nitrogen into nitrite-nitrogen so as to inhibit oxidizing reaction of nitrite-nitrogen to nitrate nitrogen, nitrite type nitrification could be achieved. For keeping the DO concentration low, the DO concentration in the reactor is measured by a DO sensor and the flow volume for aeration is controlled on the basis of the measurement.
In an experimental apparatus of small volume, precise control of DO concentration would be available so as to achieve the nitrite type nitrification. In actual water treatment apparatus, however, distribution of DO concentration would occur in the reactor where the aeration is conducted, and in general, DO sensor has a difficulty in accurate continuous measurement. For this reason, in DO concentration control of actual apparatus, it is impossible to evenly control the DO concentration in the reactor at a lower level such as in 0.1 mg/L order over a long period for reliable nitrite type nitrification so that a part of nitrite would be oxidized into nitric acid due to excess aeration.
JP 2003-10883A described that nitrite type nitrification can be stably and reliably conducted by adjusting the flow volume for aeration in such a manner that the concentration of residual ammonium-nitrogen in the reactor or effluent of the reactor is kept at 20 mg/L or more.
In case of controlling only the concentration of ammonium-nitrogen in the effluent of nitrification process as the aforementioned JP 2003-10883A, concentration ratio between ammonia and nitrite in the nitrified liquid is not controlled.